SANAA, Yemen  Yemeni security officials said that four Shiites were killed when unknown assailants fired a rocket-propelled grenade into a gathering of worshippers observing the holy day of Ashoura.

Officials said the attack took place Saturday in the Yemeni capital of Sanaa, near the airport road.

The assailants fired the weapon at a celebration hall where worshippers had gathered, killing four people outside. No further details were immediately available.

The officials spoke on condition of anonymity because they were not authorized to release the information.

Tensions have long existed between extremist Salafis, who are Sunni Muslims, and Shiite tribesmen known as Hawthis who have also at times battled the government. Nearly 200 people died in fighting late last year between Salafis and Hawthis in northern Saada province.